Sirenian and Other Marine Mammals:Evolution...

5

Sirenian and Other MarineMammals: Evolutionand Systematics

5.1. Introduction

The mammalian order Sirenia,or sea cows, includes two extant families, the Trichechidae(manatees) and the Dugongidae (the dugong). The name Sirenia comes from mermaidsof Greek mythology known as sirens. Sirenians have a fossil record extending from theearly Eocene (50 Ma) to the present (Figure 5.1). Manatees include three living speciesand are known from the early Miocene (15 Ma) to the Recent in the New World tropics.The dugong is represented by a single extant species, Dugong dugon, of the Indo-Pacific.Dugongs were considerably more diverse in the past,with 19 extinct genera described anda fossil record that extends back to the Eocene. A North Pacific lineage of dugongids sur-vived into historic times and had successfully adapted to cold climates. Sirenians areunique among living marine mammals in having a strictly herbivorous diet, which isreflected in the morphology of their teeth and digestive system. The Desmostylia, theonly extinct order of marine mammals, are relatives of sirenians and are discussed here,as is the extinct marine bear-like carnivoran, Kolponomos. Other marine mammalsinclude members of two extant carnivore families, the Mustelidae (which includes thesea otter, Enhydra lutris), the Ursidae (containing the polar bear, Ursus maritimus), andthe extinct sloth family Megalonychidae (which includes the aquatic sloth lineageThalassocnus).

5.2. Origin and Evolution of Sirenians

5.2.1. Sirenians Defined

Sirenians possess relatively large stout bodies, downturned snouts, short rounded paddle-like flippers, and a horizontal tail fluke. Manatees can be readily distinguished fromdugongs by their smaller size, a rounded rather than notched tail, and a less-pronounced

89

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deflection of the snout. The latter feature enables manatees to feed at any level in thewater column rather than being obligate bottom feeders, like the dugong with its stronglydownturned snout.

The monophyly of sirenians is well established. Sirenians are united by possession ofthe following synapomorphies (Domning, 1994; Figures 5.2 and 5.3):

1. External nares retracted and enlarged, reaching to or beyond the level of the anteriormargin of the orbit. In the primitive condition, the external nares (nostrils) are notretracted.

2. Premaxilla contacts frontal. All sirenians are characterized by a premaxilla-frontalcontact. In the primitive condition, the premaxilla does not contact the frontal; insteadit contacts the nasal posteriorly.

3. Sagittal crest absent. The skull of sirenians can be distinguished from other closelyrelated mammals in lacking development of a sagittal crest.

4. Five premolars, or secondarily reduced from this condition by loss of anterior pre-molars. Early sirenians possess five premolars as did ancestral placental mammals(Archibald, 1996). This tendency was later reversed by post-Eocene sirenians, whichoften reduce the number of premolars. Ungulates show the primitive condition, posses-sion of four premolars (Thewissen and Domning, 1992).

5. Mastoid inflated and exposed through occipital fenestra. In sirenians, the mastoidis inflated and fills a large fenestra (window-like opening) in the dorsal occiput. It doesnot extend around the base of the cranium to form a flange on the ventral occiput(Novacek and Wyss, 1987). In the primitive condition seen in most mammals, there is

90 5. Sirenian and Other Marine Mammals

PleistoMioceneOligoceneEocene

Early MiddleLate Late LateMiddle Early Early

Kolponomos †

Halitheriinae †

Desmostylia †

Thalassocnus †

510152025303540455055

Ma

Enhydra

Ursus

Trichechinae

Miosireninae †Trichechidae

Carnivora

Hydrodamalinae †

DugonginaeProtosireninae †

Prorastomus †

Dugongidae

Edentata

E LPlio-cene

0

Figure 5.1. Chronologic ranges of living and extinct sirenians and other marine mammals. Ma = millionyears ago.

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continuous mastoid exposure between the horizontal basicranium and ventral (verti-cal) occiput.

6. Ectotympanic inflated and drop-like. Sirenians are distinguished by having aninflated ectotympanic (one of the bones forming the auditory bulla) that is drop-like inshape. In the primitive condition, the ectotympanic is uninflated (Tassy and Shoshani,1988).

7. Pachyostosis and osteosclerosis present in skeleton. The skeleton of sirenians dis-plays both pachyostosis and osteosclerosis, modifications involved in hydrostatic regu-lation (Domning and de Buffrénil, 1991).

5.2.2. Sirenian Affinities

Proboscideans (elephants) are usually considered the closest living relatives of sirenians(e.g., McKenna, 1975; Domning et al., 1986; Thewissen and Domning, 1992). Charactersthat unite proboscideans and sirenians include rostral displacement of the orbits withassociated reorganization of the antorbital region, strongly laterally flared zygomaticprocess of the squamosal, and incipiently bilophodont (double crested) teeth (Savageet al., 1994). Sirenians, proboscideans, and the extinct desmostylians are recognized as amonophyletic clade, termed the Tethytheria (named because early members were thoughtto have inhabited the shores of the ancient Tethys Sea; McKenna, 1975; Figure 5.2).Morphological characters supporting an alliance between tethytheres, the perissodactyls(horses, rhinos,and tapirs),and the hyracoids (hyraxes), referred to as the Pantomesaxoniaclade (Prothero etal.,1988),have been refuted (Savage etal.,1994).Molecular data removeperissodactyls from a relationship with tethytheres and hyracoids.

5.2. Origin and Evolution of Sirenians 91

Sirenia

Proboscidea

Desmostylia †

Hyracoidea

(1-7)

PAENUNGULATA

TETHYTHERIA

Figure 5.2. A cladogram depicting the relationship of sirenians and their close relatives. Numbers refer tosirenian synapomorphies, some of which are illustrated in Figure 5.3. † = extinct taxa.

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Frontal

Premaxilla

MaxillaNasal

3. Lacks sagittal crest

(a)

(b)

Premaxilla

Frontal

2. Frontal and premaxilla in contact

Figure 5.3. Sirenian synapomorphies. (a) Snout of archaic elephant, Moeritherium, in dorsal and lateralviews illustrating the lack of contact between the premaxilla and the nasals (primitive conditionof character 2) (see text for further description). (Modified from Tassy and Shoshani, 1988.)(b) Skull of the sirenian Dusisiren, in dorsal and lateral views illustrating the derived conditionof character 2, premaxilla lies in contact with nasals. (Modified from Domning, 1978.) Also vis-ible are other sirenian synapomorphies, character: l = external nares retracted and enlarged,reaching to or beyond the anterior margin of the orbit and 3 = sagittal crest absent.

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Recognition of another clade, the Paenungulata, composed of the Tethytheria andhyracoids (Novacek et al., 1988; Shoshani, 1993), is more controversial. Fischer (1986,1989) and Prothero (1993) maintained that morphological features supporting thePaenungulata can be disallowed as shared primitive characters and therefore are notindicative of relationship. These workers have argued for a closer relationship betweenhyracoids and perissodactyls. Molecular sequence data, however, strongly support thePaenungulata clade (sirenians, proboscideans, and hyracoids; Springer and Kirsch,1993; Lavergne et al., 1996; Stanhope et al., 1998; Madsen et al., 2001; Murphy et al.,2001; Scally et al., 2001).

An African clade of diverse mammals, named Afrotheria, that includes sirenians inaddition to elephant shrews, tenrecs, golden moles, aardvarks, hyraxes, and elephantshas received consistent and strong support from molecular data (e.g., Springer et al.,1997; Stanhope et al., 1998; Scally et al., 2001; Murata et al., 2003). WithinAfrotheria, interrelationships are less clear although support was found forTethytheria (i.e., sirenians + elephants), which is the sister taxon to hyraxes (Murphyet al., 2001; Murata et al., 2003). Discovery of a new family of retroposons amongAfrotheria (AfroSINES) may help to resolve relationships among this group(Nikaido et al., 2003).

5.2.3. Evolution of Early Sirenians

The earliest known sirenians are prorastomids Prorastomus and Pezosiren from earlyand middle Eocene age rocks (50 Ma) of Jamaica (Figures 5.4 and 5.5). The dense andswollen ribs of prorastomids point to a partially aquatic lifestyle, as does their occur-rence in lagoonal deposits. The hip and knee joints of Prorastomus and Protosiren(Domning and Gingerich, 1994) and the nearly complete skeleton of Pezosiren


(a)

(b)

Figure 5.4. An early sirenian, Prorastomus sirenoides, from the late early Eocene of Jamaica. (a) Skull inlateral and ventral views. (b) Reconstructed composite skeleton of Pezosiren portelli. (FromDomning, 2001.) (Unshaded areas are partly conjecture.)

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(Domning, 2001a) indicate that the earliest sirenians possessed well-developed legs(Figure 5.4).

Study of the type skull of Protosiren fraasi using CT scans (Gingerich et al., 1994)reveals small olfactory bulbs, small optic tracts, and large maxillary nerves, consistentwith the diminished importance of olfaction and vision in an aquatic environmentand consistent with enhanced tactile sensitivity of the enlarged downturned snout ofmost Sirenia. Prorastomus and Protosiren were amphibious quadrupeds and not asfully aquatic as most later sirenians. The peculiar forceps-like snouts of Prorastomusand other early sea cows suggests a selective browsing habit by analogy with extantnarrow-muzzled ungulates. Additional morphologic, ecologic, and taphonomic datasupport consideration of prorastomids as fluvatile (river) or estuarine semiaquaticherbivores (Savage et al., 1994). Middle and late Eocene dugongids in need of taxo-nomic revision include Eotheroides and Eosiren from Egypt and Prototherium fromItaly.

5.2.4. Modern Sirenians

5.2.4.1. Family Trichechidae

Some scientists as recently as the 19th century considered the manatee to be an unusualtropical form of walrus; in fact the walrus was once placed in the genus Trichechus alongwith the manatees (Reynolds and Odell, 1991). The family Trichechidae was expandedby Domning (1994) to include not only the manatees (Trichechinae) but also theMiosireninae, a northern European clade composed of two genera, Anomotherium andMiosiren. The trichechid clade as a whole appears to have been derived from late Eoceneor early Oligocene dugongids or from protosirenids (see Gheerbrant et al., 2005). Thesubfamily Trichechinae includes three living species: the West Indian manatee(Trichechus manatus), the West African manatee (Trichechus senegalensis), and


Prorastomus †

Ribodon †

Trichechus

Protosiren †

Halitheriinae †

Dugonginae

Hydrodamalinae †

TRICHECHIDAE

DUGONGIDAE

Potamosiren †

Figure 5.5. Relationships among sirenians based on morphologic data. (Modified from Domning, 1994.)† = extinct taxa.

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the Amazon manatee (Trichechus inunguis; Figure 5.6). Two subspecies of the WestIndian manatee can be distinguished on the basis of morphology and geography, theAntillean manatee, T. m. manatus, and the Florida manatee, T. m. latirostris (Domningand Hayek, 1986). Manatees are united as a monophyletic clade by features of the skull(e.g., ear region). Other derived characters include reduction of neural spines on the ver-tebrae, a possible tendency toward enlargement, and, at least in Trichechus, anteropos-terior elongation of thoracic vertebral centra (Domning, 1994).

Morphologic data supports the West African manatee and the West Indian manateesharing a more recent common ancestor than either does with the Amazon manatee(Domning, 1982; Domning and Hayek, 1986). Mitochondrial sequence data supportsclose divergence times for the three species (Parr and Duffield, 2002).

5.2.4.2. Family Dugongidae

The Family Dugongidae is paraphyletic as defined by Domning (1994). It includestwo monophyletic subfamilies, the Dugonginae and extinct Hydrodamalinae, and theparaphyletic extinct “Halitheriinae.”

The “Halitheriinae” includes the paraphyletic genera Halitherium, Eotheroides,Prototherium, Eosiren, Caribosiren, and Metaxytherium. The best known genus,Metaxytherium, was widely distributed in both the North Atlantic and Pacific duringthe Miocene. Metaxytherium had a strongly downturned snout and small upper incisortusks. Members of this lineage were most likely generalized bottom-feeding animals that


Figure 5.6. Modern manatee species. (a) West Indian manatee. (b) West African manatee. (c) Amazon man-atee. (Illustrated by P. Folkens from Reeves et al., 1992.)

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probably consumed rhizomes (root-like stems) of small to moderate sized sea grassesand sea grass leaves (Domning and Furusawa, 1995). A Caribbean and West Atlanticorigin for the genus, with subsequent dispersal to the North Pacific via the CentralAmerican Seaway and later dispersal to coastal Peru is suggested.

The extinct Hydrodamalinae includes the paraphyletic genus Dusisiren and the line-age that led to the recently extinct Steller’s sea cow, Hydrodamalis gigas (Figure 5.7).Dusisiren evolved a very large body size, decreased snout deflection, and the loss of tusks,suggesting that these animals may have fed on kelp that grows higher in the water columnthan do sea grasses (Domning and Furusawa, 1995). Steller’s sea cow, named for its dis-coverer, Georg W. Steller, a German naturalist, was a gigantic animal. It measured atleast 7.6 m in length and was estimated to weigh between 4 and 10 tons. The sea cow wasunusual in lacking teeth and finger bones and in possessing a thick, bark-like skin.Steller’s sea cow lived in cold waters near islands in the Bering Sea, in contrast to the dis-tribution of other sirenians in tropical or subtropical waters, and in prehistoric timesfrom Japan to Baja California. The ancestry of this animal involves Metaxytherium


Figure 5.7. Steller’s sea cow. (a) Left side of the body. (Illustrated by P. Folkens from Reeves et al., 1992.)(b) Lateral and dorsal views of the skull and mandible. (After Heptner, 1974.)

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and Dusisiren jordani from the Miocene of California. Dusisiren dewana, described from9-Ma rocks in Japan, makes a good structural intermediate between D. jordani andSteller’s sea cow in showing a reduction of teeth and finger bones. A penultimate stage inthe evolution of Steller’s sea cow is represented by Hydrodamalis cuestae from 3- to 8-Madeposits in California. H. cuestae lacked teeth, probably lacked finger bones, and wasvery large.

Steller unfortunately described the sea cow’s blubber, 3–4 inches thick, as tastingsomething like almond oil. Steller’s sea cow quickly became a major food resource forRussian hunters. By 1768, only 27 years after its discovery, the sea cow was extinct.Anderson (1995) proposed that the extinction of sea cows may also have been con-tributed to by a combination of predation, competition, and decline in food supplies thatoccurred when aboriginal human populations colonized mainland coastlines andislands along the North Pacific (further discussed in Chapter 12).

The subfamily to which the modern dugong belongs, the Dugonginae, includes inaddition to Dugong the following extinct genera: Bharatisiren, Corystosiren,Crenatosiren, Dioplotherium, Rytiodus, and Xenosiren. Fossil remains of this dugongidclade have been found from 15-Ma rocks in the Mediterranean, western Europe, south-eastern United States, Caribbean, Indian Ocean, South America, and the North Pacific.The most elaborate development of tusks in the Sirenia are found in later divergingdugongines such as Rytiodus, Corystosiren, Xenosiren, and Dioplotherium. These speciespossessed enlarged, blade-like, self-sharpening tusks that may have been used to dig upsea grasses (Figure 5.8). The modern dugong may have evolved large tusks for a similarreason, but now appears to use them chiefly for social interactions. The discovery of afossil dugongine in the Indian Ocean (Bajpai and Domning, 1997) was not unexpectedgiven the presence of living Dugong in that region today and it corroborates the earlier


Figure 5.8. Members of the dugong lineage illustrating differential development of the tusks.(a) Dioplotherium manigaulti. (b) Rytiodus sp. (From Domning, 1994.)

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suggestion (Domning, 1994) that the discovery of additional fossils from that regionwould lend support for an Indo-Pacific origin for the genus.

The modern dugong, Dugong dugon (Figure 5.9), is distinguished by the followingderived characters (Domning, 1994): nasals absent, constant presence in juveniles of adeciduous first incisor, frequent presence in adults of vestigial lower incisors, sexualdimorphism in size and eruption of permanent tusks (first incisor), and functional lossof enamel crowns on cheekteeth and persistently open roots of M2-3 and m2-3.

5.3. The Extinct Sirenian Relatives—Desmostylia

5.3.1. Origin and Evolution

First described on the basis of tooth fragments, the Desmostylia bear a name derivedfrom the bundled columnar shape of the cusps of the molar teeth in some taxa (Figure5.10). These bizarre animals constitute the only extinct order of marine mammals. Theywere confined to the North Pacific area (Japan, Kamchatka, and North America) dur-


Figure 5.9. Lateral view of skeleton of modern dugong and its fossil relative. (a) Dugong dugon. (Modifiedfrom Kingdon, 1971.) (b) Left side of body. (Illustrated by P. Folkens from Reeves et al., 1992.)(c) Dusisiren jordani from the late Miocene-early Pliocene of California. (From Domning,1978.)

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ing the late Oligocene and middle Miocene epochs (approximately 33–10 Ma). Knownfossils represent at least 6 genera and 10 species, all hippo-sized amphibious quadrupedsthat probably fed on marine algae and sea grasses in subtropical to cool-temperatewaters (see Figure 5.10; Barnes et al., 1985; Inuzuka et al., 1995; Clementz et al., 2003).

Basal desmostylians are represented by Behemotops from the middle or late Oligoceneof North America and Japan (Domning et al., 1986; Ray et al., 1994). Cornwallius, a laterdiverging genus, is known from several eastern North Pacific late Oligocene localities.Paleoparadoxia is a Miocene genus known on both sides of the Pacific. Sexual dimor-phism in this species is suggested based on cranial and dental differences (Hasegawa

5.3. The Extinct Sirenian Relatives–Desmostylia 99

(a)

(b) (c)

(d)

Figure 5.10. Representative desmostylans. (a) Restored skeleton of Paleoparadoxia tabatai (FromDomning, 2002). (b) Skull and mandible of Desmostylus hesperus. (From Domning, 2001b.)(c) Lower molar of Desmostylus in lateral and occlusal aspect. (Modified from Vanderhoof,1937.) (d) Restored skeleton of Desmostylus. (From Domning 2001.)

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et al., 1995). A skeleton with skull from Point Arena, California was described as a newspecies, Paleoparadoxia weltoni (see Clark, 1991). Another new species ofPaleoparadoxia has been reported from southern California and Mexico (Barnes andAranda-Manteca, 1997). Desmostylus, the most specialized and best represented genusof the order, is found widely in Miocene coastal deposits of the North Pacific.

A phylogenetic analysis of desmostylians strongly supports a clade comprisingDesmostylus, Cornwallius, Paleoparadoxia, and Behemotops as consecutive sister taxa(Clark, 1991; Ray et al., 1994; Figure 5.11). Synapomorphies that unite desmostyliansinclude lower incisors transversely aligned, the presence of an enlarged passage presentthrough the squamosal from the external auditory meatus to roof of skull, roots of thelower first premolar fused, and paroccipital process elongated. Desmostylians are mostclosely related to proboscideans (elephants) on the basis of several characters of the lowermolars and ear region,with sirenians forming the next closest sister group (Ray etal., 1994).

Reconstructions of the skeleton and inferred locomotion of desmostylians have beencontroversial as recently reviewed by Domning (2002) and have included resemblancesto sea lions, frogs, and crocodiles (e.g., Inuzuka, 1982, 1984, 1985; Halstead, 1985).Studies by Domning (2002) indicate that desmostylians had a more upright posture sim-ilar to that seen in some ground sloths and calicotheres. Locomotion in the water was byforelimb propulsion resembling polar bears. Dental morphology is varied, and laterdiverging species show adaptations for an abrasive diet, probably one that contained gritmixed with plant material scooped from the sea bottom or shore. A stable isotope studyof tooth enamel from Desmostylus suggests that this taxon spent time in estuarine orfreshwater environments rather than exclusively marine ecosystems and likely foragedon sea grasses as well as a wide range of aquatic vegetation (Clementz et al., 2003).

5.4. The Extinct Marine Bear-Like Carnivoran, Kolponomos


The large extinct carnivoran species Kolponomos clallamensis was originally describedon the basis of an essentially toothless, incompletely preserved snout of middle Mioceneage from Clallam Bay, Washington. Study of this specimen together with new materialfrom coastal Oregon has resulted in the description of a second species, K. newportensis(Figure 5.12; Tedford et al., 1994). Kolponomos had a massive skull with a markedlydownturned snout and broad, crushing teeth.


SIRENIA

PROBOSCIDEA

Behemetops †

Palaeoparadoxia †

Cornwallius †

Desmostylus †

Vanderhoofius †

Figure 5.11. Relationships among desmostylians and related taxa. (Modified from Domning, 2001b.) † = extinct taxa.

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The relationship of Kolponomos to other carnivores has been problematic.Originally this genus was questionably assigned to the Procyonidae, a family of ter-restrial carnivores that includes raccoons and their allies. Study of additional speci-mens, including a nearly complete skull and jaw with some postcranial elements, hassupported recognition of Kolponomos as an ursoid, most closely related to membersof the extinct paraphyletic family Amphicynodontidae, which includesAmphicynodon, Pachycynodon, Allocyon, and Kolponomos (Tedford et al., 1994).Kolponomos and Allocyon are hypothesized as the stem group from which thePinnipedimorpha arose (Figure 5.13). Shared derived characters that link Allocyon,Kolponomos, and the pinnipedimorphs include details of the skull and teeth (Tedfordet al., 1994).

Kolponomos was probably coastal in distribution, because all specimens havebeen discovered in near-shore marine rocks. The crushing teeth would have beensuited to a diet of hard-shelled marine invertebrates. Kolponomos probably fedon marine invertebrates living on rocky substrates, prying them off with the incisorsand canines, crushing their shells, and consuming the soft parts as sea otters oftendo. Kolponomos represents a unique adaptation for marine carnivores; its modeof living and ecological niche are approached only by the sea otter (Tedford et al.,1994).

5.4. The Extinct Marine Bear-Like Carnivoran, Kolponomos 101

Figure 5.12. Line drawing of the skull and lower jaw of Kolponomos newportensis from the early Mioceneof Oregon. Original 25 cm long. (From Tedford et al., 1994.)

Ursidae

PINNIPEDIMORPHA

AMPHICYNODONTIDAE

Amphicynodon †Pachcynodon †Allocyon †

Kolponomos †

Enaliarctos †

Figure 5.13. Relationships among Kolponomos and related taxa. (Modified from Tedford et al., 1994.)

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5.5. The Extinct Aquatic Sloth, Thalassocnus natans


In 1995, an aquatic sloth, Thalassocnus natans (Muizon and McDonald, 1995; Muizon1996), represented by an abundance of associated complete and partial skeletons, wasreported from early Pliocene marine rocks of the southern coast of Peru (Figure 5.14).Since that discovery four additional species of the Thalassocnus lineage have beendescribed from the late Miocene-late Pliocene (McDonald and Muizon, 2002; Muizonet al., 2003; Muizon et al., 2004a). The aquatic sloth lineage spans over 4 Ma and wasapparently endemic to Peru. Thalassocnus is a nothrotheriid ground sloth on the basis ofa number of diagnostic cranial, dental, and postcranial features.

As previously known these sloths were medium to giant-sized herbivores with terrestrialor arboreal habits. As judged from its morphology and the paleoenvironment of the local-ity where these sloths have been recovered, Thalassocnus occupied an aquatic habit. Thetail probably was used for swimming and a ventrally downturned premaxilla expanded atthe apex suggests the presence of a well-developed lip for grazing. An increase in massive-ness of the dentition and associated changes in the skull and mandible to permit crushingand grinding suggests that thalassocnines were grazers and fed primarily on sea grasses(Muizon etal., 2004b).The morphological similarity of thalassocnines and desmostylians(i.e., elongate, spatulate rostra) raises the intriguing possibility that these animals were theecologic homologues of desmostylians in the South Pacific (Domning, 2001b).

5.6. The Sea Otter, Enhydra lutris

Although sea otters (Figure 5.15) are the smallest marine mammals, they are the largestmembers of the Family Mustelidae, which includes 70 species of river otters, skunks,weasels, and badgers, among others. The generic name of the sea otter is from the Greekenhydris, for “otter,” and the specific epithet is from the Latin lutra, for “otter.” Threesubspecies of sea otter are recognized based on differences in morphology as well as dis-tribution: Enhydra l. lutris (Linnaeus, 1758) inhabits the Kuril Islands, the east coast ofthe Kamchatka Peninsula, and the Commander Islands; Enhydra 1. kenyoni (Wilson


(a)

(b)

5 cm

Figure 5.14. Aquatic sloth, Thalassocnus natans from the early Pliocene of Peru. (a) Skull. (b) Lower jaw indorsal and lateral views. (From Muizon et al., 2003.) (Courtesy of C. de Muizon.)

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et al., 1991) ranges from the Aleutian Islands to Oregon; and Enhydra 1. nereis (Merriam,1904) had a historic range from northern California to approximately Punta Abrejos,Baja California.

Based on a cranial morphometric analysis, individuals of E. l. lutris are characterizedby large wide skulls with short nasal bones. Specimens of E. 1. nereis have narrow skullswith a long rostrum and small teeth, and usually lack the characteristic notch in the pos-torbital region found in most specimens of the other two subspecies. Specimens of E. 1.kenyoni are intermediate to the other two but do not possess all characters and havelonger mandibles than either of the other two subspecies (Wilson et al., 1991).

5.6. The Sea Otter, Enhydralutris 103

Figure 5.15. Sea otter, Enhydra lutris. (a) Ventral view of body. (Illustrated by P. Folkens in Reeves et al.,1992.) (b) Skull in dorsal, lateral, and ventral views and lower jaw in lateral view. (FromLawlor, 1979.)

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The modern sea otter Enhydra arose in the North Pacific at the beginning of thePleistocene, about 1 to 3 Ma and has not dispersed since that time. There are records ofEnhydra from the early Pleistocene of Oregon (Leffler, 1964) and California (Mitchell,1966; Repenning, 1976). One extinct species, Enhydra macrodonta (Kilmer, 1972), hasbeen described from the late Pleistocene of California.

The closest living relative of Enhydra are other lutine otters Lutra (Eurasian and spot-ted neck otters), Aonyx (short clawed otter), and Amblonyx (small clawed otter) based onseparate and combined analysis of mitochondrial and nuclear sequence data (Koepfliand Wayne, 1998, 2003; Figure 5.16). The morphological analysis of extant mustelids byBryant et al. (1993) differed in allying the giant otter Pteronura with other lutrines includ-ing Enhydra (see Figure 5.16). In a phylogenetic analysis that included both the living seaotter and related extinct taxa, Berta and Morgan (1985) proposed that there were two lin-eages of sea otters:an early diverging lineage that led to the extinct genus Enhydriodonanda later diverging lineage that led to the extinct giant otter Enhydritherium and the extantsea otter Enhydra (see Figure 5.16). Enhydriodon is known only from Africa and Eurasia,with three well-described species. In addition, there are several more poorly known spec-imens from Greece, England, and east Africa that have provisionally been assigned to thegenus. All of this material is of late Miocene/Pliocene age. It is not known if Enhydriodonlived in marine or freshwater habitats or both. However, they were as large or larger thanmodern sea otters and had similarly well-developed molariform dentitions (Repenning,1976). Enhydritherium is known from the late Miocene of Europe and the lateMiocene/middle Pliocene of North America. Two species of Enhydritherium aredescribed: E. lluecai from Spain and E. terraenovae from Florida and California.Enhydritherium is united with Enhydra based on dental synapomorphies.

An incomplete articulated skeleton of Enhydritherium terraenovae was describedfrom northern Florida (Figure 5.17; Lambert, 1997). The depositional environment ofthis site, which is located a considerable distance from the coast, indicates that E. terra-novae frequented large inland rivers and lakes in addition to coastal marine environ-ments. Enhydritherium was similar in size to Enhydra, with an estimated body mass ofapproximately 22 kg. The unspecialized distal hind limb elements and heavily developedhumeral muscles of Enhydritherium strongly suggest that, contrary to Enhydra, this ani-mal was primarily a forelimb swimmer. With its more equally proportioned forelimbsand hind limbs, Enhydritherium was almost certainly more effective at terrestrial loco-


Pteronura

Lutra maculicollis

Enhydra

Lontra

Lutra lutra

Amblonyx

Aonyx

Pteronura

Enhydra

Lutra

Arctonyx

Meles

Aonyx

(a) (b)

Figure 5.16. Relationships of Enhydra and related taxa. (a) Morphological data (Bryant et al., 1993). (b)Molecular data (Koepfli and Wayne, 2003).

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motion than Enhydra. The thickened cusps of the upper fourth premolars of E. terra-novae and their tendency to be heavily worn suggest that these otters, like Enhydra, con-sumed extremely hard food items such as molluscs (Lambert, 1997).

5.7. The Polar Bear, Ursus maritimus


Polar bears are the only species of bear that spend a significant portion of their lives inthe water. The generic name for the polar bear, Ursus, is the Latin word for bear, and itsspecific epithet, maritimus, refers to the maritime habitat of this species. The previoussuggestion that the polar bear (Figure 5.18) might represent a separate genus, Thalarctos,because of its adaptation to aquatic conditions and its physical appearance is not sup-ported. Ursus maritimus has a fossil record limited to the Pleistocene (Kurtén, 1964).Analysis of combined nuclear and mitochondrial sequence data (Yu et al., 2004) cor-roborate a sister group relationship between brown and polar bears (Zhang and Ryder,1994; Talbot and Shields, 1996; Waits et al., 1999). Molecular data support divergence ofpolar bears from brown bears, Ursus arctos,1–1.5 Ma (Yu et al., 2004), which is approxi-mately 10 times older than the fossil record (.07–.1 Ma; Kurtén, 1968).

5.8. Summary and Conclusions

The monophyly of sirenians is widely accepted and elephants are considered theirclosest living relatives. Sirenians, elephants, and extinct desmostylians form a mono-phyletic clade, the Tethytheria, that is part of a larger, diverse mammal clade, theAfrotheria. Sirenians are known in the fossil record from approximately 50 Ma. Earlysirenians were fluvatile or estuarine semiaquatic herbivores with functional hindlimbs. Manatees are likely derived from dugongids. An extinct lineage of dugongidsled to the recently extinct Steller’s sea cow that was cold adapted for life in the BeringSea, in contrast to other members of this lineage distributed in tropical or subtropicalwaters. The hippopotamus-like desmostylians (33–10 Ma) have the distinction ofcomposing the only extinct order of marine mammals. The large extinct bear-like

5.7. The Polar Bear, Ursus maritimus 105

Figure 5.17. Extinct giant otter, Enhydritherium terranovae from the late Miocene of Florida, skull andlower jaw in lateral view. Original 16 cm long. (From Lambert, 1997.)

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carnivoran Kolponomos is now recognized as more closely related to amphicynodon-tine ursids and pinnipedimorphs rather than its previous allocation to the raccoonfamily. The range of adaptation of sloths, formerly known to have only terrestrial andarboreal habits, was extended based on discovery of a diverse lineage of aquatic slothThalassocnus. The modern sea otter appears to have evolved in the North Pacific 1–3Ma. Among fossil sea otters is Enhydritherium, which likely frequented large rivers andlakes as well as coastal marine environments. The most recently diverging lineage ofmarine mammals, the polar bear, appears to have been derived from brown bearsbetween .5 and 1 Ma.


Melursus ursinus(Sloth bear)

Ursus americanus(American black bear)

Ursus thibetanus(Asiatic black bear)

Ursus arctos(Brown bear)

Helarctos malayanus(Sun bear)

Ursus maritimus(Polar bear)

Figure 5.19. Relationships of polar bears and their relatives. (From Yu et al., 2004.)

Figure 5.18. Polar bear, Ursus maritimus. (a) Right side of body. (b) Lateral and dorsal views of skull andlateral view of lower jaw. (From Hall and Kelson, 1959.)

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5.9. Further Reading

Sirenian phylogeny is detailed in Domning (1994, 2001b) and a popular account of theevolution of manatees and dugongs can be found in Reynolds and Odell (1991). Fora summary of the evolution and phylogeny of desmostylians see Domning (2001b,2002). A description of the bear-like carnivoran Kolponomos is provided in Tedford et al.(1994). For descriptions of the aquatic sloth see Muizon and MacDonald (1995),McDonald and Muizon (2002), and Muizon et al. (2003). Sea otter evolution isreviewed by Berta and Morgan (1985) and Lambert (1997). Bear phylogeny, especiallythe molecular evidence, is discussed by Yu et al. (2004) (Figure 5.19).

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